The invention relates to a circuit arrangement for operating at least two discharge lamps, comprising
input terminals for connection to a supply voltage source, PA1 switching means coupled to the input terminals for generating a high-frequency voltage from a supply voltage delivered by the supply voltage source, PA1 a control circuit for generating a high-frequency signal for rendering the switching means conducting and non-conducting with high frequency, PA1 a first load branch provided with two ends which are coupled to the switching means, with first inductive means, with first capacitive means, and with terminals for accommodating a first discharge lamp, and PA1 a second load branch provided with two ends which are connected to the ends of the first load branch, with second inductive means, with second capacitive means, and with terminals for accommodating a second discharge lamp.
Such a circuit arrangement is known from German Patent Application DE 4039161 A1. Two discharge lamps in parallel load branches can be operated with the known circuit arrangement. A problem which arises in general in the operation of discharge lamps in parallel load branches is the control of the luminous fluxes of the discharge lamps. The luminous fluxes of the discharge lamps may, for example, differ strongly because the discharge lamps have different electrical properties. To counteract this problem, the known circuit arrangement is provided with a balance transformer. A first winding of this balance transformer forms part of the first load branch, and a second winding forms part of the second load branch. It is possible by means of this balance transformer to minimize a difference between the luminous fluxes of the two discharge lamps. The balance transformer, however, is a voluminous and comparatively expensive component. It was found in practice, moreover, that the balance transformer is less capable of eliminating a difference between the luminous fluxes in proportion as the luminous fluxes of the two discharge lamps are set for a lower value. As a result of this, a comparatively great difference between the luminous fluxes may be present in the case of low luminous flux set values of the discharge lamps in spite of the presence of the balance transformer. It was also found in practice that the lowest adjustable luminous fluxes of the two discharge lamps are strongly dependent on parasitic capacitances in the circuit arrangement.